R：如何更改data.frame中的值

Question

> dummy <- data.frame(X  = c(1, 2, 3, 4, 5, 5, 2, 6, 7, 2), Y = c(3, 2, 1, 4, 5, 6, 7, 3, 4, 2))
> dummy
   X Y
1  1 3
2  2 2
3  3 1
4  4 4
5  5 5
6  5 6
7  2 7
8  6 3
9  7 4
10 2 2

I have a data.frame that consists of values from 1 to 7. I want to change the 1's to 7's (and vice versa), 2's to 6's (and vice versa), 3's to 5's (and vice versa), and the 4's will stay as 4's. Ie essentially I want to 'reverse' the numbers. I thought about writing a for loop to iterate over each value in each column and use ifelse statements, but how can I change, say, the 7's to 1's and the 1's to 7s simultaneously?

Answer 1

Considering all the pairs of numbers you want to switch have a sum of 8, you can subtract your original data frame from 8 and all the values should be reverted as you want, so you can just do 8 - dummy :

dummy = 8 - dummy
dummy
#   X Y
#1  7 5
#2  6 6
#3  5 7
#4  4 4
#5  3 3
#6  3 2
#7  6 1
#8  2 5
#9  1 4
#10 6 6

Answer 2

match is the right generic way to do this - it will work even when you can't find a nice simple mathematical operation:

First set up key and value vectors, where the ith entry of key you want to replace with the corresponding entry of value :

key = 1:7   # key to look up (current value)
value = 7:1 # desired value corresponding to key

dummy$newX = value[match(dummy$X, key)]
dummy$newY = value[match(dummy$Y, key)]

#    X Y newX newY
# 1  1 3    7    5
# 2  2 2    6    6
# 3  3 1    5    7
# 4  4 4    4    4
# 5  5 5    3    3
# 6  5 6    3    2
# 7  2 7    6    1
# 8  6 3    2    5
# 9  7 4    1    4
# 10 2 2    6    6

You could, of course, directly overwrite X and Y - I keep them both here to demonstrate that it worked.

Answer 3

Making a little more generic:

max(dummy) + min(dummy) - dummy
   X Y
1  7 5
2  6 6
3  5 7
4  4 4
5  3 3
6  3 2
7  6 1
8  2 5
9  1 4
10 6 6